Figure 10.41 shows the root-locus diagram for a feedback amplifier whose open-loop
response is characterized by three poles. As indicated, increasing the loop gain from zero
moves the highest-frequency pole outward while the two other poles are brought closer
together. As A0β is increased further, the two poles become coincident and then become
complex and conjugate. A value of A0β exists at which this pair of complex-conjugate poles
enters the right half of the s plane, thus causing the amplifier to become unstable.